The MayoClinic website has a tab called "Health Information". On the Health Information pages you will find an overwhelming amount of information about diseases and conditions and their treatment.
You can use the Symptom Checker to identify the condition from which you're suffering.
You can use the Test and Procedures tab to find out how a procedure works and what it diagnoses. An extensive drug library can be found on this website that explains the conditions each drug is used to correct.
The website also gives tips on living a healthy lifestyle and how to administer first aid.